The cerebellum, a brain structure found in all vertebrates, is thought to guide motor learning based on sensory inputs. The overall goal of this project is to understand, at a fundamental level, how sensory information is integrated by Purkinje neurons. Purkinje neurons are of interest because they are the point of convergence for the two main input streams to the cerebellum, the mossy fiber/parallel fiber pathway and the inferior olive/climbing fiber pathway. In addition, Purkinje neurons are the cerebellum's sole output and are thought to play a key role in modulating motor and cognitive activity. Two-photon microscopy, which allows optical sectioning and imaging deep in brain tissue, will enable the observation of dendritic processing in brain slices, and observation of many neurons at once in living animals. Patterned photolysis of caged neurotransmitters and second messengers will allow spatially complex dendritic activity to be manipulated with millisecond resolution. This proposal will test the following three ideas. (1) Complex patterns of granule cell inputs are integrated by Purkinje cells according to two kinds of rules, a dendritic integration rule for local changes and a somatic rule for determining firing output. (2) Purkinje cells have single synapse-level mechanisms for detecting the timing of granule cell activity relative to two kinds of instructive input: climbing fiber activity and local dendritic depolarization. (3) Purkinje cells are activated in vivo in subdendritic and multicellular patterns to generate signals that drive output firing and synaptic plasticity. Taken together, these three ideas contribute to a model in which patterns of activity in granule cells undergo plasticity, and can themselves drive plasticity, to shape Purkinje cell output.